PHASE
But the question is what actually is a phase ? And how can we define or identify it ?
Well, here are some other definitions of phase,
- A phase is a macroscopically homogeneous body of matter.
- A phase is a region of space throughout which all physical properties of material are essentially uniform.
- A phase is a form of matter which have uniform chemical and physical properties. It is a distinctive form of matter such as solid, liquid, gas and plasma.
Ok, but are the terms state of matter and phase same ? And what do we mean by matter ?
Matter : Anything that have mass and occupies space.
State of matter : One of the distinct forms in which matter can exist.
Note : States of matter are phases but matter can exist in different phases yet having the same state of matter.
Eg. Pure Iron-(Fe) have 3 states of matter solid, liquid and gas. But it has 5 phases α-Fe, γ-Fe, δ-Fe, liquid and gas. Here α-Fe, γ-Fe and δ-Fe have same state of matter which is solid.
Uhhhh!!! It's so confusing to understand, right ? NO! Let's break down the first definition of phase.
' A phase is a region of material that is chemically uniform, physically distinct and mechanically separable.'
- Condition-1 ( Chemically uniform ) = same chemical properties which includes reactivity, type of bonds, toxicity, co-ordination number, enthalpy, etc.
- Condition-2 ( Physically distinct ) = same crystal structure, same state of matter, same physical properties like density, volume, magnetization, composition, etc.
- Condition-3 ( Mechanically separable ) = there should be clear macroscopic interface between two distinct phases.
Whenever we want to identify phases, we will consider these three conditions. But it's not necessary that all of them should be satisfied. Any one of them will do.
Eg-1 Water-(H2O) have 3 phases ice, liquid and vapor or in other words solid, liquid and gas.
But how can we say that all three are different and why we consider them as separate phases ? Because after all they all are water-(H2O).
First of all they all have different state of matter ( condition-2 ) that's why we can directly say that they are phases. Secondly, all three states are mechanically separable at macroscopic level ( condition-3 ) that's why we can define them as separate phases.
Eg-2 Iron-(Fe) have 5 phases α-Fe, γ-Fe, δ-Fe, liquid and gas; as we discussed earlier.
Firstly, α-Fe, γ-Fe, δ-Fe have solid state and others have liquid and gaseous state ( condition-2 ) that's why we can directly identify them as phases.
Secondly, solid state phases α-Fe, γ-Fe, δ-Fe have different physical properties such as magnetization and crystal structure ( condition-2 ).
Thirdly, all of them are mechanically separable at macroscopic level ( condition-3 ).
α-Fe is BCC & magnetic upto 768 degree C
γ-Fe is FCC & non-magnetic
δ-Fe is BCC & non-magnetic
Phase Equilibrium & Number of Phases
Many compositions will form a uniform single phase, but depending upon temperature and pressure even a single substance may exist into two or more distinct phases. Within each phase, the properties are uniform but between the two phases properties differ.
For a given composition, only certain number of phases are possible at a given temperature and pressure. Number of possible phases are difficult to calculate and are calculated only by experiments.
Between two phases in equilibrium there is a narrow region where the properties are not that of either phases. Although this region may be very thin, it can have significant and easily observable effects.
When a substance undergoes a phase transition it either takes up or release energy.
Important Note : Solid solutions and substances are considered as phases but mixtures are not.
Let's test your concepts
Which of these are phases and which are not ?
1. α-iron
2. Liquid iron
3. Pearlite
4. Austenite
5. Cementite
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Have you decided your answers ?
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Ans : Each of them are phases except Pearlite. WHY ?
Consider the important note given above.